1. Field
The present invention relates generally to communication systems, and more specifically to a apparatus and an apparatus for scrambling information bits on a channel in a communication system.
2. Description of Related Art
Communication systems have been developed to allow transmission of information signals from an origination station to a physically distinct destination station. In transmitting information signal from the origination station over a communication channel, the information signal is first converted into a form suitable for efficient transmission over the communication channel. Conversion, or modulation, of the information signal involves varying a parameter of a carrier wave in accordance with the information signal in such a way that the spectrum of the resulting modulated carrier is confined within the communication channel bandwidth. At the destination station the original information signal is replicated from the modulated carrier wave received over the communication channel. Such a replication is generally achieved by using an inverse of the modulation process employed by the origination station.
Modulation also facilitates multiple-access, i.e., simultaneous transmission and/or reception, of several signals over a common communication channel. Multiple-access communication systems often include a plurality of remote subscriber units requiring intermittent service of relatively short duration rather than continuous access to the common communication channel. Several multiple-access techniques are known in the art, such as time division multiple-access (TDMA), frequency division multiple-access (FDMA), and amplitude modulation multiple-access (AM). Another type of a multiple-access technique is a code division multiple-access (CDMA) spread spectrum system that conforms to the “TIA/EIA/IS-95 Subscriber station-Base Station Compatibility Standard for Dual-Mode Wide-Band Spread Spectrum Cellular System,” hereinafter referred to as the IS-95 standard. The use of CDMA techniques in a multiple-access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE-ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND APPARATUS FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of the present invention.
A multiple-access communication system may be a wireless or wire-line and may carry voice and/or data. An example of a communication system carrying both voice and data is a system in accordance with the IS-95 standard, which specifies transmitting voice and data over the communication channel. A apparatus for transmitting data in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled “APPARATUS AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION”, assigned to the assignee of the present invention. In accordance with the IS-95 standard, the data or voice is partitioned into code channel frames that are 20 milliseconds wide with data rates as high as 14.4 Kbps. Additional examples of a communication systems carrying both voice and data comprise communication systems conforming to the “3rd Generation Partnership Project” (3GPP), embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), or “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems” (the IS-2000 standard).
In a multiple-access communication system, communications between users are conducted through one or more base stations. A first user on one subscriber station communicates to a second user on a second subscriber station by transmitting data on a reverse link to a base station. The base station receives the data and can route the data to another base station. The data is transmitted on a forward link of the same base station, or the other base station, to the second subscriber station. The forward link refers to transmission from a base station to a subscriber station and the reverse link refers to transmission from a subscriber station to a base station. Likewise, the communication can be conducted between a first user on one mobile subscriber station and a second user on a landline station. A base station receives the data from the user on a reverse link, and routes the data through a public switched telephone network (PSTN) to the second user. In many communication systems, e.g., IS-95, W-CDMA, IS-2000, the forward link and the reverse link are allocated separate frequencies.
An example of a data only communication system is a high data rate (HDR) communication system that conforms to the TIA/EIA/IS-856 industry standard, hereinafter referred to as the IS-856 standard. This HDR system is based on a communication system disclosed in co-pending application Ser. No. 08/963,386, entitled “APPARATUS AND APPARATUS FOR HIGH RATE PACKET DATA TRANSMISSION,” filed Nov. 3, 1997, assigned to the assignee of the present invention. The HDR communication system defines a set of data rates, ranging from 38.4 kbps to 2.4 Mbps, at which an access point (AP) may send data to a subscriber station (access terminal, AT). Because the AP is analogous to a base station, the terminology with respect to cells and sectors is the same as with respect to voice systems.
In a multiple-access communication system, communications between users are conducted through one or more base stations. A first user on one subscriber station communicates to a second user on a second subscriber station by transmitting data on a reverse link to a base station. The base station receives the data and can route the data to another base station. The data is transmitted on a forward link of the same base station, or the other base station, to the second subscriber station. The forward link refers to transmission from a base station to a subscriber station and the reverse link refers to transmission from a subscriber station to a base station. Likewise, the communication can be conducted between a first user on one mobile subscriber station and a second user on a landline station. A base station receives the data from the user on a reverse link, and routes the data through a public switched telephone network (PSTN) to the second user. In many communication systems, e.g., IS-95, W-CDMA, IS-2000, the forward link and the reverse link are allocated separate frequencies.
Some service providers may be disadvantaged by the need to deploy a predominantly voice communication system, e.g., an IS-2000 revision 0 communication system on one carrier frequency, and a HDR communication system, e.g., IS-856 communication system on a separate carrier frequency. Such a disadvantage may arose from limited spectrum availability, insufficient customer base, business objectives or other reasons known to one of ordinary skills in the art. Consequently, the “3rd Generation Partnership Project 2” (3GPP2) has started to develop a standard for enabling voice and high rate data communication on one carrier frequency. Such a system is described in a draft known as Release C of cdma2000 (hereinafter referred to as Draft).
The Draft specifies a apparatus of allowing voice users to utilize the forward channels defined by the IS-2000 standard. In the excess capacity, data users are served on contain up to two Forward Packet Data Channels (F-PDCH). Therefore, the F-PDCH is used to transmit data user information from a base station to the mobile stations. The user data are transmitted in packets called encoder packets. Each encoder packet is encoded, scrambled, and interleaved. Some or all of the interleaved symbols then form sub-packets, which are modulated and de-multiplexed into a variable number of in-phase and quadrature pairs of parallel streams. The modulation can comprise a quadrature phase-shift keying (QPSK), eight phase-shift keying (8-PSK), or sixteen Quadrature Amplitude Modulation (16-QAM). The number of streams varies in accordance with the demands of the voice users and data users. Each parallel streams is encoded with a distinct 32-ary Walsh function. The Walsh-coded symbols of all the in-phase streams are summed to form a single in-phase stream, and the Walsh-coded symbols of all the quadrature streams are summed to form a single quadrature stream. The resulting in-phase and quadrature streams are quadrature spreaded and transmitted. The F-PDCH is shared by mobile stations based on time multiplexing, thus transmitting information to one specific mobile station at a time.
As follows from the above description, the modulation format of the F-PDCH is variable. Consequently, to enable a mobile station to receive and process the F-PDCH, the base station also transmits control messages on a Forward Packet Data Control Channel (F-PDCCH) transmitted in parallel with the F-PDCCH as illustrated in FIG. 1. The control message comprises information required by the subscriber station for correct reception of the F-PDCH, e.g., an identifier of a subscriber station for which the F-PDCH is intended to, a modulation format of the F-PDCH, and other information as specified in the Draft. Because the amount of data that the F-PDCH transmits varies, e.g., with channel conditions, amount of data to be transmitted, the time for transmission (t2-t1) varies. The transmission time can be expressed in units of slots, a slot being a 1.25 ms unit. In accordance with the Draft specifications, the slot-format comprises one-slot, two-slots or four-slots. The slot-format of the F-PDCCH is the same as the slot-format of the F-PDCH.
In one embodiment, the F-PDCCH utilizes control messages of length of 21 bits. To assure consistency of control message content, a content quality indicator is computed, and appended to the control message. In one embodiment, the content quality indicator comprises a cyclic redundancy check (CRC). A conceptual structure of an exemplary F-PDCCH control message 200 is illustrated in FIG. 2. The control message 200 comprises 21 bits comprising the information bits of a control message content 202, and 8 bits comprising a quality indicator 204.
A conceptual structure of an exemplary F-PDCCH 300 is illustrated in FIG. 3. The 21 bits of input data 302, comprising the content control message, are concatenated with 8 error detection encoder bits in block 306. The bit stream is further concatenated with 8 encoder tail bits in block 308, and encoded in block 310. In one embodiment, the encoder is a convolutional encoder, well known in the art, with constraint length 9 and rate ½, ⅓, or ¼. Depending on the slot-format a particular encoding rate is selected, i.e., ½ rate for the one-slot format, ⅓ rate for the two-slot format, and ¼ rate for the four-slot format. The encoded symbols are provided to block 312, which adjusts length of the encoded symbols for further processing by puncturing/repeating some symbols to generate 48 symbols for the one-slot format, 96 symbols for the two-slot format, and 192 symbols for four-slot format control message. The symbols are provided to a block interleaver 314. The interleaved symbols are then provided to a quadrature-phase shift keying (QPSK) modulator 316. The In-phase (I) and quadrature-phase (Q) outputs of the QPSK modulator 316 are spreaded by a Walsh code (W) in spreaders 318(I) and 318(Q) and provided to a transmitter (not shown).
One of ordinary skills in the art understands that a particular embodiment of the F-PDCCH has been described for tutorial purposes. Consequently, other embodiments are contemplated. In particular, other encoders, interleavers, modulators, and spreading codes known to one of ordinary skills in the art can be used.
The control message, transmitted by the base station is received at a mobile station. The mobile station uses an inverse of the F-PDCCH processing as described in reference to FIG. 3, to recover the 21 bits comprising the control message. Because the mobile station has no information of how many slots comprises the control message, the mobile station performs the inverse of the F-PDCCH processing under the assumption that the last slot received comprises a control message that is transmitted in the one-slot format. Thus, the mobile station computes a CRC from the first 21 bits of the decoded output (ref. 204 of FIG. 2), which the mobile station interprets as the information bits, and compares the computed CRC with the last 8 bits of the decoded output, (ref. 204 of FIG. 2), which the mobile station interprets as the CRC. If the computed CRC is equal to the interpreted CRC, the mobile station declares a valid control message. If the computed CRC is not equal to the interpreted CRC, the mobile station performs the inverse of the F-PDCCH processing under the assumption that the last two slots received comprise a control message that was transmitted in the two-slot format, and determines whether the CRC checks as described in the above. If the computed CRC is equal to the interpreted CRC, the mobile station declares a valid control message. If the computed CRC is not equal to the interpreted CRC, the mobile station performs the inverse of the F-PDCCH processing under the assumption that the last four slots comprise a control message that is transmitted in the four-slot format, and determines whether the CRC checks as described in the above. If the computed CRC is equal to the interpreted CRC, the mobile station declares a valid control message. If the computed CRC is not equal to the interpreted CRC, the mobile station declares that no valid control message is found. The mobile station then waits until next slot is received and repeats the above process.
The above-described procedure may yield an incorrect determination of control message slot format. For example, assume that a control message is a two-slot message. Given the above described procedure, the mobile station performs the inverse of the F-PDCCH processing under an assumption that the control message is transmitted in a one-slot format, and interprets the first 21 bits of the decoded output as message content 202 (of FIG. 2), and the remaining 8 bits as a quality indicator 204 (of FIG. 2). Unfortunately, the content of certain control messages sometimes results in control messages that appear to be valid in a sense that, the computed CRC is equal to the interpreted CRC. Consequently, the mobile station declares a valid control message, and a false-alarm event occurs.
Because the same control message contents transmitted on the F-PDCCH are repeated from time to time, there is likelihood that the information bits comprising the message will cause repetitive false-alarm events. Therefore, there is a need in the art for a apparatus an apparatus preventing such repetitive incorrect determination of control message content to happen.
Although the background of the invention was described for pedagogical reasons in terms of a control channel of a communication system in accordance with the Draft, one of ordinary skill in the art will understand that the teaching is applicable to any system providing control messages on any control channel.